On 28 August at 14.15 John Paulo Aquino Samin will defend his doctoral thesis “Unified pH in aqueous-organic solvent mixtures and low-polarity solvents”.
Supervisors:
Prof. Ivo Leito, Institute of Chemistry, University of Tartu
Dr. Agnes Heering, Institute of Chemistry, University of Tartu
Opponent:
Professor Sébastien Thibaudeau, PhD Institut de Chimie des Milieux et Matériaux de Poitiers, Université de Poitiers, France
Summary:
pH is the most widely used parameter for describing a solution’s acidity and plays an important role in many everyday processes, from biological activities to industrial applications. While pH measurement is well-established and highly reliable in aqueous solutions, many chemical processes occur in mixtures of water and organic solvents, or in an entirely non-aqueous medium. In these environments, acidity behaves differently because hydrogen ions interact differently with each solvent. As a result, each solvent has its own pH scale, and the measured pH values are not directly comparable across different media. To overcome this limitation, the concept of a “unified pH scale” (pHabs scale) was introduced in 2010. It replaces solvent-specific reference points with a universal reference point based of an ideal proton gas under standard conditions. In this work, the unified pH scale was established for solutions in mixtures of water and organic solvents (methanol and acetonitrile), and in low-polarity solvents (1,2-dichloroethane and 1,2-difluorobenzene). This study relied on measuring the potential difference between solutions using an electrochemical technique known as differential potentiometry to determine the relative acidities of solutions. Unified acidity values were determined for 78 aqueous–organic solvent mixtures commonly used as mobile phases in liquid chromatography. The research also determined acidity values for 41 buffer solutions in low-polarity solvents, 1,2-dichloroethane and 1,2-difluorobenzene, spanning 10 orders of magnitude in acidity. These findings showed that potentiometric measurements can be performed successfully even in low-polarity solvents, opening the possibility of extending the unified pH scale to a wider range of solvents.
